Liquid compositions of fluorinated ion exchange polymers are known for use in the manufacture of ion exchange membranes, for membrane coatings containing conductive and non-conductive particles, and for many other uses. While such compositions are sometimes referred to as solutions, the compositions are generally recognized as being dispersions (i.e. colloidal suspensions) of polymer particles.
GB 1286859 (DU PONT DE NEMOURS) 23 Aug. 1972 discloses solutions of (per)fluorinated ion exchange polymers comprising sulfonate, sulfonamide or sulfonic acid groups in an organic solvent. The (per)fluorinated ion exchange polymers contain at least 14 mole % of monomer units which contain the sulfonate, sulfonamide or sulfonic acid groups. The organic solvent is at least 5% by weight soluble in water and is selected from C1-C4 alcohols, fluorocarbon alcohols, organic amides, acetone and preferably has a boiling point lower than 130° C. The reported concentration of the polymer in organic solvents other than alcohols is very low, around 1% by weight.
U.S. Pat. No. 4,433,082 (DUPONT DE NEMOURS) 21 Dec. 1984 discloses a process for dissolving (per)fluorinated ion exchange polymers having —SO3M functional groups (wherein M is H, Na, K, NR4) having an equivalent weight in the range of 1025-1500 g/eq in an aqueous-alcoholic liquid medium. The preferred medium is a mixture of 20-90% by weight of water and 10-80% by weight of an alcohol (C1-C4) optionally in the presence of other solvents miscible in water. The process is carried out at a temperature of from 180 to 300° C. in a closed vessel obtaining two liquid phases having a different density which are separated.
EP 1004615 A (AUSIMONT SPA) 31 May 2000 discloses a process to prepare a solution and/or dispersion containing (per)fluorinated ion exchange polymers comprising —SO3M functional groups (wherein M is H, Li, Na, K, NR4) in a monophasic ternary mixture comprising 0.1%-50% by weight water, 50%-99% by weight of a C1-C4 alcohol and 0.1%-40% by weight of a fluoro(poly)oxyalkylene having one hydrogen atom in at least one fluorinated end group. The process is carried out at a temperature comprised between room temperature and 150° C.
In all of these documents alcohol-based liquid phases are used for the preparation of concentrated solutions. The use of alcohol-based liquid compositions is however undesirable for some applications. For example, when the (per)fluorinated ion exchange polymer is used as a binder for the preparation of catalyst-containing electrodes residual traces of alcohols may cause side reactions and may even represent a fire hazard. Also, alcohol-based liquid compositions of (per)fluorinated ion exchange polymers are generally characterized by a high viscosity that renders them difficult to use in the preparation of membranes, for instance by impregnation of porous substrates.
Alcohol-free liquid compositions comprising fluorinated ion exchange polymers, namely aqueous liquid compositions comprising up to 10% by weight of a (per)fluorinated ion exchange polymer having an equivalent weight in the range of 1025-1500 g/eq were disclosed in U.S. Pat. No. 4,433,082 (DUPONT DE NEMOURS) 21 Dec. 1984. U.S. Pat. No. 6,150,426 (DU PONT DE NEMOURS) 21 Nov. 2000 also discloses liquid compositions containing either an aqueous liquid medium or a non-aqueous liquid medium with 0.5 to 50% by weight of a (per)fluorinated ion exchange polymer having well defined solid particle size distribution, said compositions being substantially free of water miscible alcohols. The aqueous liquid compositions are obtained by means of a dissolution process at temperatures between 150 and 350° C. in a pressurized vessel, in the presence of water or, optionally in the presence of water and from 0.5 to 75% by weight of a water immiscible organic solvent which separates from the liquid composition at the end of the process. The non-aqueous liquid compositions are obtained by re-dispersion of the solids obtained by drying the aqueous liquid composition of the ion exchange polymer.
Water-based liquid compositions are generally unsuitable for the preparation of composite membranes, for instance by coating or impregnation of a porous support due to their reduced ability to wet surfaces, in particular fluorinated surfaces.
Thus, a need still exists for liquid compositions comprising a sufficiently high concentration of a fluorinated ion exchange polymer in organic solvents other than alcohols that can be prepared without requiring the use of high pressures and temperatures. In particular, a need still exists for liquid compositions having an appropriate balance between the concentration of the fluorinated ion exchange polymer, the viscosity of the liquid composition and its surface tension to be suitably employed for the preparation of composite membranes.